One of the major challenges operators face in deployment of 3G networks, such as UMTS, is the ability to provide broadband speeds to users throughout the entire cell. In typical network deployments, the users at the cell edge often get served with much smaller data rates compared to users closer to the base station. While proper link budget planning and cell site placement can ensure that users at the cell edge are able to achieve a minimum desired data rate (e.g., 128 kbps average throughput), the reality is that cell site locations from existing 2G deployments will be used, making such minimum desired data rates difficult to achieve.
Exacerbating the situation even further, the 2G network (e.g., GSM) may have been operating in the 850 MHz band, whereas the 3G network is operating in both the 850 MHz band as well as the 1900 MHz band. The significantly increased path loss experienced at the higher carrier frequency limits the data rates that users can see at the cell edge even further; this is especially a problem in the uplink as the mobiles typically transmit at relatively low power levels (e.g., 125 mW).
Given that wireless operators may own spectrum in both a lower frequency band (such as 850 MHz) as well as a higher frequency band (1900 MHz) in a given market, wireless operators may want to assign and/or move mobiles between the multiple frequency bands or carriers. Unfortunately, the current mechanism for switching carriers (sometimes referred to as carrier handoff or inter-frequency handoff) is a lengthy process, taking up to 5 seconds to carry out. This type of delay is not acceptable in mobile wireless environments with potentially high vehicle speeds.